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rate substantially greater than the rate of rotation about the a,@-carbon-carbon single bond, In accord with this mechanism, the rate of attack by hydride (cf. Table I) is facilitated by a-alkyl groups as these will stabilize the radical 111. 0 Substituents retard the reaction presumably by sterically hindering the approach of the hydride.8 (8) The authors express their thanks to the referees for helpful comments and for drawing their attention to ref l j . (9) Address correspondence to Chemistry Department, The Pennsylvania State University, University Park, Pa. 16802.
L. M. Jackman,O J. A, Hamilton, J. M. Lawlor Department of Organic Chemistry, Unicersity of Melbourne Melbourne, Victoria, Australia Receiaed December IS, 1967
Cyclodimerization of Styrene' Sir:
Table I. Composition of Styrene Dimer Fractions& Conditions
1
2
A. Radiolysis* 0.04 M styrene in benzene 3.1 1.1 0.2M,0.1~rr-BC,cinbenzene 7.4 3.7 0.2 M , 1 a t m N20, in benzene 7.2 3.3 Neat, with 0.1 % t-BC 22.8 25.9
3
4
5.3 11.1 6.2 4.1
1.1 11.0 6.6 11.2
B. Sensitized Photolysisd Sensitizere 0.01-0.02 M 0.22 0.75
0.03
C. Direct Photolysis/ 0.75 0.10 0.72 0.14
0.08 0.07
Benzene Methylene chloride
No additive 0.1 % t-BC
D. Thermal0 0.09 0.18
0.21 0.36
0.59 0.11
, ,
.
, . .
0.02 0.29
a Products: 1, cis-1,2-diphenylcyclobutane; 2, trans-1,2-diphenylcyclobutane; 3, I-phenyltetralin; 4, I-phenyI-l,2-dihydronaphthalene; composition given as mole fraction, except in radiolysis. * Cobalt-60 y irradiation a t 25'; yields a s g values X 100. t-Butylcatechol. Photosensitized (uranium glass filter); 0.2 M styrene in benzene. eAveraged results for benzophenone, xanthenone, phenyl cyclopropyl ketone, a n d anthraquinone. Direct photochemical, 0.8 M styrene, various solvents. Similar results were found in methanol, diethyl ether, a n d isohexane a t 30" or at -80". thermal, 0.2 M styrene in benzene, 110", 120 hr.
The dimerization of styrene occurs thermally, photochemically, and by ionic mechanisms, either cationic3 or a n i ~ n i c , with ~ characteristic product patterns. It is thus of potential mechanistic significance to identify dimeric radiolysis products of styrene. The benzene-sensitized radiolysis, if actuated by triplet benzene as in cis-trans isomerizations of olefins,j should correlate with the photosensitized dimerization. In this communication it is shown that the correlation has limited validity. The argument rests mainly on the previously uncharacterized stereochemistry of diphenylcyclobutane formation in variously initiated dimerizations. The new measurements (Table I) were made at low conversions (0.1-0.5 % dimer) to minimize subsequent isomerization of the radiation-sensitive and thermally labile diphenylcyclobutanes. In dilute solutions at these conversions, high polymer formation was not qualitatively noticeable or detectable by analysis for unreacted monomer. The CI6fractions were analyzed, with bibenzyl as internal standard, by chromatography on lightly loaded QF-1 columns at 135". y Irradiations were performed at a dose rate of 4 X 1 O I 7 eV/g min with total doses of 4 X 10'9 to 4 X lozoeV/g. Ultraviolet irradiations utilized a high-pressure mercury lamp and appropriate filters with solutions in sealed Pyrex cells at 30". Yields of cis- and trans-1,2-diphenylcyclobutanes (1 and 2, respectively) were satisfactorily reproducible but the relative yields of I-phenyltetralin (3) and l-phenyl-l,2-dihydronaphthalene(4) in the thermal and radiolytic reactions were somewhat erratic despite care to exclude air. Addition of t-butylcatechol, considered as a possible antioxidant, actually increased the yield of 4 and decreased 3. The yield of l-phenylnaphthalene was appreciable in direct photolysis (7 % of total dimer) but small in other cases and unobserved in sensitized photolysis. In radiolysis, two unidentified components, eluting between 1 and 3, contributed together less than 10%.
Dimer yields in the radiolysis of dilute solutions are low compared with g values for benzene triplet production of 5 and higher from cis-trans isomerizations,6 and even in neat styrene the total dimer represents a g of less than 1. The dimerization in dilute solution is more than the fractional energy absorbed by monomer would account for and can thus be described as benzene sensitized. Cationic mechanisms are evidently not involved, since the characteristic open-chain olefins and indan derivatives are absent, but an anionic, or solvated electron, mechanism is admissible for at least part of the observed I-phenyltetralin.7 The products of primary interest are 1 and 2 since they are the main products in sensitized photolysis and presumably arise by an excited-state mechanism. There is disagreement in the isomer yields; cis predominates in radiolysis by a factor of 2 whereas photolysis, with any of the sensitizers tried, favors trans by a factor of 3.4. In direct photolysis, with singlet excitation by absorption in the long-wavelength band of styrene, the situation is again reversed, cis being favored under widely different conditions by a factor This is evidently an example, not the first,8 of -6. of dimerization through strong cis-pairwise complexing of the excited singlet. A cisltrans ratio in radiolysis that is midway between the two photochemical situations is the basis for concluding that cyclobutane formation in the radiolysis of benzene solutions occurs about equally by triplet and singlet excitation. The thermal formation of diphenylcyclobutanes, not allowedg as a one-step process, may be related to photosensitized dimerizations through a common inter-
(1) Work performed under the auspices of the U. S . Atomic Energy Commission. (2) F. R. Mayo,J. Am. Chem. SOC.,90, 1289 (1968). (3) B. B. Carson, J. Dorsky, J. E. Nickel, W. M. Kutz, and H. I. Thayer, J . Org. Chem., 19, 17 (1954). (4) A . Zwierzak and H. Pines, ibid., 28, 3392 (1963). (5) E. Fischer, H. P. Lehmann, and G. Stein, J . Chem. Phys., 45, 3905 (1966), and earlier literature therein cited.
(6) M. A. Golub and C. L. Stephens, J . Ph.rs. Chem., 70, 3576 (1966). (7) The supposedly anionic dimerization, in refluxing styrene with potassium ?-butoxide, to 1-phenyltetralin only4 was under conditions that would allow thermal dimerization and isomerization of primary products. (8) D. 0. Cowan and R. L. Drisko, J . Am. Chem. Soc., 89, 3068 (1 967). (9) R. Hofmann and R. B. Woodward, ibid., 87, 2046 (1965).
Journal of rhe American Chemical Society 1 90:7
March 27, 1968
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mediate, possibly the FlorylO diradical. If so, the stereochemical pattern should be the same; presently available data, not strictly comparable as to temperatures, show order-of-magnitude agreement with a thermal translcis ratio of 2.2 compared with 3.4 in sensitized photolysis, A further mechanistic correlation accommodates by-products in the 1-phenyltetralin series in photolysis and radiolysis since a 1,6 closure would form the Diels-Alder dimer that, in Mayo's2 thermal mechanism, is the required precursor. Conversely, in the thermal reaction, this dimer is a possible precursor also in cyclobutane formation since, on bond energy considerations, it is intermediate between styrene and diradical. The still hypothetical Diels-Alder dimer is doubtless a sensitive substance but conceivably could be made in demonstrable amount by low temperature sensitized photolysis of styrene. Although the radiolysis experiments here described have revealed no products of unequivocally ionic origin, further experimentation should be performed under conditions of intensive drying since, in high polymer formation, this condition is now known to favor ionic mechanisms. (10) P. J. Flory, J . Am. Chem. Soc., 59, 241 (1937). (1 1) F. W. Burns, R. M . O'Connor, and D . C. Pepper, J . Polym. Sci., Part B, 5 , 1059 (1967), and earlier cited literature.
Weldon G. Brown Chemistry Division, Argonne National Laboratory Argonne, Illinois 60439 Received December 18. 1967
The Structure of Batrachotoxinin A, a Novel Steroidal Alkaloid from the Colombian Arrow Poison Frog, Phyllobates aurotaenia
Sir : Earlier studies1,*on the potent venom from the Columbian arrow poison frog3 had indicated the presence of three major alkaloids, batrachotoxin, batrachotoxinin A, and batrachotoxinin B. Batrachotoxin, the most active cardiotoxin known (EDjo, intravenous in anesthetized dogs,4
